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SubjectRe: Linux 2.4.10-pre11
On Wed, Sep 19, 2001 at 03:21:09PM -0400, Alexander Viro wrote:
> int fd = open("/dev/ram0", O_RDWR);
> ioctl(fd, BLKFLSBUF);
> ioctl(fd, BLKFLSBUF);

here it is the fix below.

actually it also notably fixes a subtle security problem with the
ramdisk. The fact is that we are asked to read/write from the same
pagecache page that also was the destination of the I/O (ramdisk is
zerocopy), so we actually need a new bitflag to know if this a page
secure, this way we know if we need to bother hiding its contents or
not, if it's a secure page we don't need to do anything. It is a zero
cost bitflag for the nonramdisk case so it shouldn't be a problem and I
cannot imagine a better fix.

Now the only bit left in the ramdisk is a race condition in rmmod, I can
now see why you were converting rd_inode to rd_bdev :).

--- 2.4.10pre11aa1/include/linux/mm.h.~1~ Thu Sep 20 01:20:38 2001
+++ 2.4.10pre11aa1/include/linux/mm.h Thu Sep 20 03:57:25 2001
@@ -282,6 +282,7 @@
#define PG_checked 12 /* kill me in 2.5.<early>. */
#define PG_arch_1 13
#define PG_reserved 14
+#define PG_secure 15

/* Make it prettier to test the above... */
#define Page_Uptodate(page) test_bit(PG_uptodate, &(page)->flags)
@@ -295,6 +296,9 @@
#define TryLockPage(page) test_and_set_bit(PG_locked, &(page)->flags)
#define PageChecked(page) test_bit(PG_checked, &(page)->flags)
#define SetPageChecked(page) set_bit(PG_checked, &(page)->flags)
+
+#define PageSecure(page) test_bit(PG_secure, &(page)->flags)
+#define SetPageSecure(page) set_bit(PG_secure, &(page)->flags)

extern void __set_page_dirty(struct page *);

--- 2.4.10pre11aa1/mm/filemap.c.~1~ Tue Sep 18 15:39:51 2001
+++ 2.4.10pre11aa1/mm/filemap.c Thu Sep 20 03:59:00 2001
@@ -621,7 +621,7 @@
if (PageLocked(page))
BUG();

- flags = page->flags & ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_dirty | 1 << PG_referenced | 1 << PG_arch_1 | 1 << PG_checked);
+ flags = page->flags & ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_dirty | 1 << PG_referenced | 1 << PG_arch_1 | 1 << PG_checked | 1 << PG_secure);
page->flags = flags | (1 << PG_locked);
page_cache_get(page);
page->index = offset;
--- 2.4.10pre11aa1/mm/shmem.c.~1~ Tue Sep 18 02:43:04 2001
+++ 2.4.10pre11aa1/mm/shmem.c Thu Sep 20 03:59:09 2001
@@ -353,7 +353,7 @@
swap_free(*entry);
*entry = (swp_entry_t) {0};
delete_from_swap_cache_nolock(page);
- flags = page->flags & ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_referenced | 1 << PG_arch_1);
+ flags = page->flags & ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_referenced | 1 << PG_arch_1 | 1 << PG_secure);
page->flags = flags | (1 << PG_dirty);
add_to_page_cache_locked(page, mapping, idx);
info->swapped--;
--- 2.4.10pre11aa1/mm/swap_state.c.~1~ Tue Sep 18 02:43:04 2001
+++ 2.4.10pre11aa1/mm/swap_state.c Thu Sep 20 03:59:20 2001
@@ -70,7 +70,7 @@
BUG();

/* clear PG_dirty so a subsequent set_page_dirty takes effect */
- flags = page->flags & ~(1 << PG_error | 1 << PG_dirty | 1 << PG_arch_1 | 1 << PG_referenced);
+ flags = page->flags & ~(1 << PG_error | 1 << PG_dirty | 1 << PG_arch_1 | 1 << PG_referenced | 1 << PG_secure);
page->flags = flags | (1 << PG_uptodate);
add_to_page_cache_locked(page, &swapper_space, entry.val);
}
--- 2.4.10pre11/drivers/block/rd.c Tue Sep 18 02:42:23 2001
+++ 2.4.10pre11aa1/drivers/block/rd.c Thu Sep 20 04:22:44 2001
@@ -188,11 +188,23 @@

static int rd_blkdev_pagecache_IO(int rw, struct buffer_head * sbh, int minor)
{
- struct address_space * mapping = rd_inode[minor]->i_mapping;
+ struct address_space * mapping;
unsigned long index;
- int offset, size, err = 0;
+ int offset, size, err;

- if (sbh->b_page->mapping == mapping) {
+ err = -EIO;
+ /*
+ * If we did BLKFLSBUF just before doing read/write,
+ * we could see a rd_inode before the opener had a chance
+ * to return from rd_open(), that's ok, as soon as we
+ * see the inode we can use its i_mapping, and the inode
+ * cannot go away from under us.
+ */
+ if (!rd_inode[minor])
+ goto out;
+ err = 0;
+ mapping = rd_inode[minor]->i_mapping;
+ if (sbh->b_page->mapping == mapping && PageSecure(sbh->b_page)) {
if (rw != READ)
SetPageDirty(sbh->b_page);
goto out;
@@ -217,7 +229,7 @@

hash = page_hash(mapping, index);
page = __find_get_page(mapping, index, hash);
- if (!page && rw != READ) {
+ if (!page) {
page = grab_cache_page(mapping, index);
err = -ENOMEM;
if (!page)
@@ -227,18 +239,40 @@
}

index++;
- if (!page) {
- offset = 0;
- continue;
- }

if (rw == READ) {
src = kmap(page);
+
+ if (!PageSecure(page)) {
+ clear_page(src);
+ SetPageSecure(page);
+ }
+
src += offset;
dst = bh_kmap(sbh);
} else {
dst = kmap(page);
dst += offset;
+
+ if (!PageSecure(page)) {
+ unsigned long addr, start, end;
+
+ /* clear around */
+ addr = (unsigned long) dst;
+ if (addr & ~PAGE_CACHE_MASK) {
+ start = addr & PAGE_CACHE_MASK;
+ end = addr;
+ memset((char *) start, 0, end - start);
+ }
+ addr = (unsigned long) dst + count;
+ if (addr & ~PAGE_CACHE_MASK) {
+ start = addr;
+ end = (addr + PAGE_CACHE_SIZE - 1) & PAGE_CACHE_MASK;
+ memset((char *) start, 0, end - start);
+ }
+ SetPageSecure(page);
+ }
+
src = bh_kmap(sbh);
}
offset = 0;
@@ -321,6 +355,13 @@
struct block_device * bdev = inode->i_bdev;

down(&bdev->bd_sem);
+ /*
+ * We're the only users here, protected by the
+ * bd_sem, but first check if we didn't just
+ * flushed the ramdisk.
+ */
+ if (!rd_inode[minor])
+ goto unlock;
if (bdev->bd_openers > 2) {
up(&bdev->bd_sem);
return -EBUSY;
@@ -328,8 +369,16 @@
bdev->bd_openers--;
bdev->bd_cache_openers--;
iput(rd_inode[minor]);
+ /*
+ * Make sure the cache is flushed from here
+ * and not from close(2), somebody
+ * could reopen the device before we have a
+ * chance to close it ourself.
+ */
+ truncate_inode_pages(rd_inode[minor]->i_mapping, 0);
rd_inode[minor] = NULL;
rd_blocksizes[minor] = rd_blocksize;
+ unlock:
up(&bdev->bd_sem);
}
break;


Andrea
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